1. Field of the Invention
This invention relates to an improved method for forming an isostress-contoured die, such as may suitably be employed for forming isostress-contoured sheets from thin metal.
2. Description of the Prior Art
A number of industrial heat exchange applications have created a demand for lightweight, inexpensive heat exchangers formed from thin-walled heat exchange channel elements. In U.S. Pat. No. 3,757,856 to L. C. Kun a lightweight, potentially inexpensive heat exchange element is disclosed which can be used to fabricate a heat exchanger of exceptional strength and excellent heat transfer performance characteristics. The Kun heat exchanger comprises an array of parallel channels formed of thin heat conductive walls which have on their surface isostress contours with uniformly disposed unidirectional wall-supporting projections formed from the wall. An isostress surface is mathematically described in the Kun patent, as representing a surface having a multiplicity of continuously curved isostress-contours thereon; each contour is devoid of flat segments and resembles the curved contour of a shear-free "soap bubble membrane".
The aforementioned Kun patent teaches a method of forming a stamping die with which heat exchange walls can be fabricated from thin sheet metal. This method involves fabricating a block having on its surface multiple vertical projection supports forming a pattern and being dimensionally sized to correlate to the pattern and size of the wall-supporting projections desired in an isostress-contoured surface. Upwarding extending sides are provided around the edges of the block, thereby providing a recess or cavity which contains the vertical supports. The so-formed cavity is connected to depressurizing means so that when a flexible material is tensionally secured across the top of the cavity and also contacting and supported by the vertical projected supports, the depressurizing means can be operated to force the unsupported portion of the flexible material into the cavity while the vertical projected supports prevent deflection of the supported portion of the flexible material, thereby causing the flexible material to assume isostress-contours between and surrounding the supported portions which correspond to the wall-supporting projections. Subsequently, a form-setting material, i.e., a thermosetting resin, can be deposited onto the flexible material and when properly cured, the depressurizing means can be deactivated. The cured material having the isostress-contoured surface with substantially uniformly disposed unidirectional wall-supporting projections is then ready to be used as a die.
Despite its relative simplicity, the method for forming a die disclosed in the Kun patent is characterized by several severe deficiencies. For example, the form-setting material used to fashion the die does not provide a long life in the severe metal stamping service for which it is employed. In addition, the isostress surface thus formed by the membrane deformation method may deviate from the ideal isostress surface as a consequence of variations of the elastic modulus over the extent of the membrane and excessive localized stretching of the membrane in the areas approximate to the vertical projection supports.
Concerning the short practical service life for the die formed by the membrane deformation method of Kun, some improvement can be obtained by employing the resin die as a template to fabricate a steel die by a single point electric discharge machining (EDM) operation. Nonetheless, polishing and heat annealing of the thus-formed steel die, or, alternatively, depending on material hardness, polishing and stress-relieving of the die, are required to finish the die surface and the finishing process is expensive and subject to cumulative dimensional variations.
The resin die formed by the membrane deformation method of Kun can also be used as a pattern for the precision casting of a steel die; however, such casting requires polishing, finishing and heat treating which causes undesirable sizing variations. The overall die forming process thus includes the operations of casting, polishing, finishing and heat treating. As a result the finished article suffers from the accumulation of dimensional errors in the constituent steps and it is comparatively difficult and expensive to maintain such accumulation of dimensional errors at a suitably low level.
As an alternative to the above described die forming methods, a metal die can be machined employing a milling or cutting machine capable of numerical control in which the various metal cutting tools are coupled with a computer programmed according to the equation for the three-dimensional isostress surface. The computer in this system directs the three-dimensional manipulation of a single point metal removal tool. This process again is expensive and, in common with the above described methods, involves a polishing and finishing step, with the disadvantages attendant the utilization of such polishing and finishing steps, as already described.
Accordingly, it is an object of the present invention to provide a method of fabricating a metal die whose surface is a substantially isostress-contoured surface, by simple and inexpensive machining steps which produce an isostress surface to a predictable close tolerance, thereby minimizing the amount of polishing and hand finishing which is required to produce the finished die.
Other objects and advantages of the invention will be apparent from the ensuing disclosure and appended claims.